The dome vent damper can help you get a closer date of manuf/sales. From your pic of it, it appears that your in the '78-'79-'80 period. As I recall, if the damper has an 'A' or 'B' stamped into it, then it's either a '79 ('A'-code) or '80 ('B'-code). If it's missing the stamped letter, then it's probably a '78. SJ